A tiny userspace mutex class designed to fit in a single integer. Similar to `std::atomic_ref`, but compatible with older versions of C++ standard.

tiny_futex.hpp

#if defined(WIN32) || defined(_WIN32) || defined(__WIN32__) || defined(__NT__)
#include <Windows.h>
#endif

namespace ashvardanian {
  

#if defined(WIN32) || defined(_WIN32) || defined(__WIN32__) || defined(__NT__)

/**
 *  @brief  Tiny Fast Userspace Mutex for Windows,
 *          designed to fit in a single integer.
 */
class tiny_futex_t {
    using slot_t = volatile LONG;

    slot_t flag_;

  public:
    inline tiny_futex_t(slot_t flag = 0) noexcept : flag_(flag) {}
    inline ~tiny_futex_t() noexcept {}
    inline void unlock() noexcept { InterlockedExchange(&flag_, 0); }

    inline bool try_lock() noexcept {
        slot_t raw = 0;
        return InterlockedCompareExchange(&flag_, 1, raw);
    }

    inline void lock() noexcept {
        slot_t raw = 0;
        InterlockedCompareExchange(&flag_, 1, raw);
    }
};

#else

/**
 *  @brief  Tiny Fast Userspace Mutex using GCC and Clang builtins, 
 *          designed to fit in a single integer.
 */
class tiny_futex_t {
    using slot_t = signed;
    slot_t flag_;

  public:
    inline tiny_futex_t(slot_t flag = 0) noexcept : flag_(flag) {}
    inline ~tiny_futex_t() noexcept {}
    inline void unlock() noexcept { __atomic_store_n(&flag_, 0, __ATOMIC_RELEASE); }

    inline bool try_lock() noexcept {
        slot_t raw = 0;
        return __atomic_compare_exchange_n(&flag_, &raw, 1, true, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED);
    }

    inline void lock() noexcept {
        slot_t raw = 0;
    lock_again:
        raw = 0;
        if (!__atomic_compare_exchange_n(&flag_, &raw, 1, true, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED))
            goto lock_again;
    }
};

#endif  
  
}